Electrical switch for primary control system for furnaces

ABSTRACT

An electrical switch for use in a primary control system for furnaces, the switch including a mounting base and first and second bimetallic blades, one end portion of the first blade being fixed to the base while the other end portion of the first blade is fixed to one end portion of the second blade. Stop means is provided which is carried by the other end portion of the second blade, and a pair of contact members are fixed to the base, the contact members being in electrical contact with each other when one of such members is in engagement with the stop means. Means is provided for heating the first blade to cause the stop means to be disengaged from said one contact member responsive to bending of the first blade, the directions of bending of the first and second blades responsive to a change in temperature being such that an increase in temperature of the second blade will tend to counteract deflection of the stop means responsive to an equivalent increase in the temperature of the first blade.

United States Patent [191 Bauer 1 1 ELECTRICAL SWITCH FOR PRIMARYCONTROL SYSTEM FOR FURNACES [76] Inventor: Frederick T. Bauer, 1587 S.

Washington Ave., Holland, Mich. 49423 [22] Filed: May 24, 1973 [21]Appl. No.: 363,325

Related U.S. Application Data [60] Continuation of Set. Nov 109,487,Jan. 25, 1971, abandoned, which is a division of Ser. No. 866,528,

Oct. 15, 1969, Pat. No. 3,624,407.

[52] U.S. Cl 337/72, 337/75, 337/77, 337/82, 337/99 [51] Int. Cl. t.H0lh 71/16 [58] Field of Search 337/55, 56, 66, 82, 71, 337/72, 75, 77,95, 96, 99, 101, 102, 336, 378

[56] References Cited UNITED STATES PATENTS 2,293,382 8/1942 Case et a1.337/66 2,325,511 7/1943 Heintzen.... 337/99 UX 2,558,908 7/1951 Paige337/72 X 2,952,757 9/1960 Ellenberger 337/77 X 3 205,327 9/1965 Moorheadet a1. 337/71 X 3 238.3 3/1966 Randolph et a], 337/101 3.601.736 8/1971Sepe 337/101 3.674.952 7/1972 Ellenberger 337/77 X 1 Mar. 25, 1975FOREIGN PATENTS OR APPLICATIONS 845,309 8/1960 United Kingdom 337/75Primary Examiner-Arthur T. Grimley Attorney, Agent, or FirnzMalco1m R.McKinnon 157] ABSTRACT An electrical switch for use in a primary controlsystem for furnaces, the switch including a mounting base and first andsecond bimetallic blades, one end portion of the first blade being f xedto the base while the other end portion of the first blade is fixed toone end portion of the second blade. Stop means is provided which iscarried by the other end portion of the second blade, and a pair ofcontact members are fixed to the base, the contact members being inelectrical contact with each other when one of such members is inengagement with the stop means. Means is provided for heating the firstblade to cause the stop means to,

be disengaged from said one contact member responsive to bending of thefirst blade, the directions of 1 Claim, 8 Drawing Figures BRIEF SUMMARYOF THE INVENTION This invention relates to primary control means forfurnaces and the like and, more particularly, to an improved primarycontrol incorporating an improved control circuit and an improvedbimetallic type safety switch and effective to control a burner of afurnace.

In the past, primary controls have been utilized to control the burnersin furnaces and such prior primary controls have incorporated relativelybulky, heavy and complicated main motor relays and safety circuit relayswhich function to control the furnace burner. Prior primary controls forfurnaces have become increasingly complicated and expensive in theattempts to control reliably the furnace burners and such prior primarycontrols have many complicated, interrelating parts which are heavy,bulky and expensive and incorporate numerous moving parts with theresult that prior primary controls have a relatively short life and areoften plagued with service problems.

An object of the present invention is to overcome the aforementioned aswell as other disadvantages of prior primary controls for furnaces andto provide an improved control which eliminates the necessity ofproviding main motor and safety circuit relays. which provides improvedfurnaceburner control, and which is extremely reliable in operation.

Another object of the invention is to provide an improved primarycontrol for furnaces which is relatively compact and light in weight,which operates with relatively little heat generation, and which isreadily adaptable to meet the control requirements of various types offurnaces.

Another object of the invention is to provide an improved primarycontrol for furnaces which incorporates improved and greatly simplifiedmeans for controlling furnace burner operation.

Another object of the invention is to provide an improved primarycontrol for furnaces which is economical and commercially feasible tomanufacture, assemble and test with mass production labor and methodsand which is durable and efficient in operation.

Another object of the invention is to provide an improved primarycontrol for furnaces incorporating improved means assuring fail-safeoperation of the unit and associated furnace burner.

The above as well as other objects and advantages of the presentinvention will become apparent from the following description, theappended claims and the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagramof a primary control embodying the present invention;

FIG. 2 is a perspective view of a primary control structure embodyingthe present invention;

FIG. 3 is a top view of the base of the primary control illustrated inFIG. 2;

FIG. 4 is a top view of the circuit board of the primary controlillustrated in FIG. 2, showing the compo nents of the circuitillustrated in FIG. 1 assembled thereon;

FIG. 4A is a bottom view of the structure illustrated in FIG. 4 andindicating the circuitry embodied thereon;

FIG. 5 is a partial cut away section of the primary control illustratedin FIG. 2 and illustrating the bimetallic type safety switch embodiedtherein with the contacts thereof in the open position;

FIG. 6 is a partial cut away section of the structure illustrated inFIG. 5, showing the same during the resetting operation thereof; and

FIG. 7-is a partial cut away section of a portion of the structureillustrated in FIG. 5, showing the safety switch contacts in the closedcondition.

DETAILED DESCRIPTION Referring to the drawings, and more particularly toFIG. 1 thereof, the circuitry for a primary control, generallydesignated 10, embodying the present invention is schematicallyillustrated therein. As shown in FIG. 1, the primary control 10 iscomprised of a step down transformer 12 having a primary winding 14 andsecondary windings l6 and 18, the primary winding 14 being adapted to beconnected to a conventional source of volt alternating current while, inthe embodiment of the invention illustrated, each of the secondarywindings l6 and 18 of the isolated step-down transformer has a potentialof approximately 8 volts AC. The primary control 10 also includes abimetallic type safety switch generally designated 20, includingnormally closed contacts 21 and 22 and a heater coil 23; a conventionalthermostat generally designated 24; a reed switch, generally designated25, having contacts 26 and 28 and independent concentrically wound coilsRC1 and RC2, the contacts 26 and 28 being enclosed within a hermeticallysealed glass envelope 30 while the coils RC1 and RC2 are concentricallywound therearound; a triac O1 and a silicon controlled rectifier SCRI.The primary control 10 also includes a cadmium sulfide flame detector32, resistors R1, R2, R3, R4, R5, R6, R8, and R9; capacitors, C2 and C3;a potentiometer R7 and diodes D1 and D2. As shown in FIG. I, the primarycontrol 10 is connected to and adapted to control a conventional burner34 ofa furnace (not shown). The terminal 35 of the burner 34 isconnected to the source of power by the lead Ll while the terminal 36 ofthe burner is connected to the terminal 37 of the triac Q1 by the leadL2, the terminal 38 of the triac Q1 being connected to the source ofpower by the lead L3 through a fuse F1. The contact 28 of the reedswitch 25 is connected by the lead L4 to the lead L2 through theresistor R8 while the contact 26 is connected to the gate 39 of thetriac Q1 by the lead L5, the resistor R9 and capacitor C3 beingconnected across the leads L2 and L3 by the leads L4 and L6 to protectthe triac Q1.

The terminal 40 of the secondary winding 16 is connected to the contact21 of the safety switch 20 by the lead L7 while contact 22 of the safetyswitch is connected to the terminal 42 of the thermostat 24 by the leadL8, the terminal 44 of the thermostat being connected by the lead L9through the resistors R5 and R6 and the coil RC1, to the terminal 46 ofthe diode D1. The terminal 48 of the diode D1 is connected to the centertap 50 of the secondary windings of the transformer 12 by the lead L10.The capacitor C2 is connected across the resistor R6 and the coil RC1 ofthe reed switch 25 by the lead L11 while the resistor R3 is connectedbetween the leads L9 and L10 by the lead L12 as illustrated in FIG. 1.The terminal 44 of the thermostat 24 is also connected to the terminal52 of the diode D2 by the lead L13 through the resistor R4, the terminal54 of the diode D2 being connected to the center tap 50 of thetransformer 12 by the lead L10. As shown in FIG. 1, the contact 22 ofthe safety switch 20 is connected .to the terminal 56 of the siliconcontrolled rectifier SCRl through the heater coil 23 and the coil RC2 ofthe reed switch 25, the potentiometer R7 being connected across the coilRC2. The terminal 58 of the silicon controlled rectifier SCRl isconnected to the center tap 50 of the transformer 12 by the leads L14and L10 while the gate 60 of the rectifier SCRl is connected to theterminal 52 of the diode D2 by the lead L and to the terminal 62 of thesecondary winding 18 of the transformer by the lead L16 through theresistor R1, the cadmium sulfide flame detector 32 and the resistor R2.

The above described components are preferably mounted on one side of thecircuit board 65 as illustrated in FIG. 4 and the entire circuitstructure is integrated as by soldering as illustrated in FIG. 4A. Inuse, the circuit board 65 is mounted on a base 66 within a housing 67.

Referring in greater detail to the components of the primary control 10,the safety switch illustrated schematically in FIG. 1 and illustratedstructurally in FIGS. 4, 5, 6 and 7 is comprised of a mounting base 68adapted to be secured to the circuit board 65 of the primary control,the circuit board 65 being formed of plastic or other suitablenon-conducting material having sufficient strength to withstand theforces exerted thereon in carrying the components of the primary controlas illustrated in FIG. 4 and the various electrical leads as illustratedin FIG. 4A. The mounting base 68 is generally U-shaped in side view, asillustrated in FIGS. 4, 5, 6 and 7, and includes a substantially flat,horizontally extending web portion 72 having upwardly projecting flangeportions 74 and 76 at the opposite ends thereof. A support bracket 78 isprovided which is riveted or otherwise fixed to the upper end of theflange portion 74 as viewed in FIG. 5, and the support bracket 78carries a bimetallic blade 80 one end of which is fixed to the free endof the bracket 78 while the opposite end of the bimetallic blade 80carries a substantially flat bimetallic blade 82 the free end of whichis provided with an integral flange 84 that projects angularlydownwardly from the flat body portion of the blade 82. A pair of contactblade springs 86 and 88 are provided having struck out portions 90 and92, respectively, which function as the normally closed contacts 21 and22 in the safety switch 20. The blade 86 includes a generally upwardlyprojecting portion 94 which carries the struck out portion 90 and anintegral generally horizontally extending portion 96 which is secured tothe web portion 72 of the mounting base 68 by a rivet 100, the portion96 being electrically insulated from the mounting base 68 by anelectrical insulator (not shown) disposed between the portion 96 and theweb portion 72 of the mounting base 68. The blade 88 includes agenerally upwardly projecting portion 102 which carries the struck outcontact portion 92 and an integral generally horizontally extendingportion 104 which is also secured to the web portion 72 of the mountingbase 68 as by a rivet 106.

As shown in FIG. 7, in the normally closed condition, the upper endportion 102 of the blade 88 is adapted to engage thefree edge of thedownwardly projecting portion 84 of the bimetallic blade 82 whereby thecontact portion 92 is maintained in engagement with the contact portionof the blade 86, an adjusting screw 108 being provided which threadablyengages a projecting portion 109 on the flange 74 to permit initialadjustment of the blade 82. 1

The bimetallic blade 80 is adapted to bend upwardly upon an increase intemperature whereas the bimetallic blade 82 is adapted to benddownwardly upon an increase in temperature, the bimetallic blade 82 thusacting as a compensator for variations in ambient temperature. Theheater element 23 surrounds the bimetallic blade element 80, the heaterelement 23 functioning to heat the bimetallic element 80 as will bedescribed hereinafter in greater detail whereby the bimetallic elements80 and 82 move upwardly so that the upper end portion of the bladespring 88 disengages from the free end of the flange portion 84 of thebimetallic blade 82 and moves to the right, as viewed FIGS. 5, 6 and 7so as to open the contacts 21 and 22 of FIG. 1.

Means are provided for resetting the safety switch and returning thecontacts 21 and 22 to their normally closed position. Such means iscomprised of an elongate plunger'l10 which is preferably formed ofplastic or other suitable non-conducting material and which is supportedfor sliding movement by the upwardly projecting flange portion 76 of themounting base 68 and by the end wall 112 of the housing 67 of theprimary control 10, the flange portion 76 and the wall 112 of thehousing 67 having openings 116 and 118, respectively, which function asbearing supports for the plunger 110. The plunger is of steppedconstruction and includes a button portion 120 and an enlarged annularflange portion 122 which is joined to the button portion 120 by aradially extending flange 124 adapted to seat in a recess 125provided'in the end wall 112 of the housing. A reduced diameter portion126 is also provided on the plunger 110 as well as a further reduceddiameter portion 128, the portion 126 being adapted to pass through theopening 116 defined by the flange 76 while the portion 128 is adapted topass through an opening defined by the blade 88 and to engage the upperend portion 94 of the blade 86. A spring 132 is provided one end portionof which en gages the flange 76 of the mounting base 68 while theopposite end of the spring 132 engages the flange 124 of the plunger 110so as to bias the plunger toward the wall 112 of the housing 67. Withsuch a construction, when the contact portions 90 and 92 are in the opencondition, as illustrated in FIG. 5, the contact portions 90 and 92 maybe moved to the normally closed position by manually pushing the buttonportion 120 of the plunger 110 to the left as viewed in FIG. 5. Theshoulder 127 intermediate the portions 126 and 128 of the plunger 110then engages the blade element 88 while the portion 128 of the plunger110 passes through the opening 130 in the blade 88 and engages the blade86 so that the blade 86 and 88 move to the left and assume the positionillustrated in FIG. 6, the upper end of the blade 88 engaging the outersurface of the portion 84 of the bimetallic blade 82 so as to move thebimetallic blade 82 upwardly during such operation due to the cam actionof the portion 84 of the blade 82 whereby the components assume theposition illustrated in FIG.

6. Closing the contact portions 90 and 92 is effected upon release ofmanual pressure on' the end of the button portion 120 of the plunger110, the spring 132 functioning to return the plunger 110 to theinoperative position illustrated in FIG. 7 with the flange 124 of theplunger abutting the wall 112 of the housing 67. The spring blades 86and 88 then move back to the right as viewed in FIG. 7, and since theupper end portion 102 of the blade 88 is stopped by the free end of thedownwardly inclined portion 84 of the blade element 82, the contactportions 90 and 92 close and remain closed until the switch is againopened by upward movement of the blade 82.

The rectifier SCRl is a conventional silicon controlled rectifier andmay, for example, carry a rating of approximately four amperes. Thethermostat 24 may be of any desired or conventional construction whilethe reed switch 25 is preferably of the type disclosed in the applicantscopending application entitled Switch Construction. As previouslymentioned such a switch is comprised of a pair of contacts 26 and 28carried by reeds hermetically sealed within a glass envelope 30. Such areed switch also includes the electrically insulated, independentlywound concentric coils RC1 and RC2, the magnetic fluxes of such coilsbeing additive when in phase. The reed switch 25 preferably has a verylarge differential between pull-in and dropout ampere turns or coilpower. By way of example, the reeds preferably will pull in at about 60ampere turns, but will not drop out until below 20 ampere turns, a ratioof at least 3 to 1. In the embodiment of the invention illustrated inFIG. 1 the maximum power to the coil RC1 is well below that required topull-in the reed switch and close the contacts 26 and 28. The power is,however, enough to hold the reed switch contacts 26 and 28 closed oncepull-in has been established, due to the very large differential. Thereed switch coil RC2, on the other hand, has sufficient power whencombined with RC1 to pull in the reed switch. Since reed switches arevery fast they are capable of followingan alternating current voltage toopen or close 60 or 120 times per second. To avoid this opening andclosing and the associuted wear, the diode D1 and capacitor C2 areprovided. The diode D1 is preferably a 200 milliampere diode whichsupplies half wave rectified current to the capacitor C2 to establish aDC supply for the reed switch coil RC1. The capacitor C2 is preferably a47 microfarad volt DC capacitor. The diode D1 and capacitor C2 functionto form a DC supply for the holding coil RC1 so that flux is alwayspresent on the coil RC1 when the thermostat calls for heat. This flux isvery small however. With such a construction and since relatively smallcurrent passes through the contacts 26 and 28, such contacts are veryreliable over a relatively long life.

The triac O1 is a bidirectional thyrister which may be gate triggeredfrom a blocking to conducting state for either polarity of appliedvoltage, and is preferably mounted in a recess 135 defined by anintegral flange 136 projecting outwardly from one side of the base 66and functioning to isolate the other components of the control 10 fromthe heat generated by the triac Q1. The resistors R1 and R2 arepreferably carbon resistors having ratings of 150 ohms and 560 ohms,respectively, one-half watt, the purpose of the resistor R1 being toprevent the accidental destruction of the diode D1, transformer 12 orsilicon controlled rectifier SCRl by a serviceman in the field. In thisconnection the resistors R1, R2, R5, and R9, the diode D2 and thecapacitor C3 are all provided in the primary control 10 solely for thepurpose of protecting other components and to protect against erroneouswiring in the field. The resistors R1, R2, R5 and R9, the diode D2 andthe capacitor C3 are thus not essential to the basic circuitperformance.

Typical values for the components in the control system described aboveare as follows:

SCRl 4 AMP Silicon controlled rectifier D] 200 Ma diode D2 200 Ma diodeRl Carbon resistor I50 ohms, 20%, H2 watt R2 Carbon resistor 500 ohms, i20%, H2 watt R3 Wirewound resistor 20 ohms, :t 20%, 5 watt R4 Carbonresistor 3300 ohms, i 2071, V2 watt R5 Carbon resistor 47 ohms, i 20%, Awatt R6 Wirewound resistor 680 ohms, i 20%, l watt R7 Wirewoundpotentiometer l ohm, t 20%, 2 watt R8 Carbon resistor 82 ohms, i 20%, /2watt R9 Carbon resistor 82 ohms, i 20%, V2 watt C2 Capacitor 47 mfd l5VDC C3 Capacitor, 22 mfd 200 V Mylar foil It will be understood,however, that these values may be varied depending upon the particularapplication of the principles of the present invention.

Assuming a basic knowledge of the triac Q1, the silicon controlledrectifier SCRl, and the cadmium sulfide flame detector 32, a typicalthermostat cycle operates in the following manner. It should be notedinitially that whenever the reed contacts 26 and 28 are closed, currentwill flow from the source of electric power through the lead L1, theburner 34, the lead L2, the resistor R8 and contacts 26 and 28, to thegate of the triac Q1 and the lead L3. When the gate ofthe triac O1 isenergized the full motor current will then pass through the triac Q1.This starts the burner and has the same effect as closing a set of relaycontacts between the lead L2 and the lead L3.

Whenever the thermostat contacts close, a continuous holding flux isestablished in the coil by the DC supply network comprised of the diodeD1 and the capacitor C2. Current also flows through the resistor R4 tothe gate of the silicon controlled rectifier SCR-l. If the cadmiumsulfide flame detector 32 registers darkness, no current can be shuntedaway from the gate 60 of the silicon controlled rectifier SCRl and SCRlwill conduct. When SCRl conducts, current also passes through thepull-in coil RC2 of the reed switch 25 and the heater 23 of the safetyswitch 20. With a flux established in the coil RC2 and the coil RC1, thereed switch contacts 26 and 28 will pull in and the triac Q1 will startthe burner. If the cadmium sulfide flame detector 32 does not registerflame, the silicon controlled rectifier SCRl will continue to conductand the safety switch 20 will open the contacts 21 and 22 due to theheating action of the heater 23 raising the bimetallic blade 82 throughthe raising of the bimetallic blade 80. It is preferred that thecontacts 21 and 22 open and lock out after approximately 15 seconds. Ifthe cadmium sulfide flame detector registers flame, then the flamedetector 32 decreases in resistance and shunts current away from thegate 60 of the rectifier SCRl. SCRl will no longer conduct, the heatingcoil 23 of the safety switch will be deenergized but the coil RC1 willcontinue to hold in the reed relay contacts 26 and 28. If the cadmiumcell 32 registers flame and for some reason the flame should go outduring the thermostat cycle, the rectifier SCRl willagain conduct andthe heating coil 23 will be energized so as to open the contacts 21 and22 into a lock-out condition. When the thermostatic conditions aresatisfied and the contacts thereof open, the coil RC1 is deenergizedthereby opening the contacts 26' and 28 and also deenergizing the triacQ1. No current is then available through the resistor R4 to energizeSCR1 even though the cadmium cell 32 registers no flame. It should alsobe understood that the same cycle would occur if the thermostat wereconnected to line voltage and placed in one leg of the transformerprimary coil.

, An important aspect of the present invention resides in the fact thatif there is a failure in the primary control 10, the primary controlwill fail in a safe condition. For example, if the silicon controlledrectifier SCRl is shorted from anode to cathode it will conduct electriccurrent supplied by the secondary winding 16 of the transformer. Thecadmium sulfide flame detector 32 will have no effecton the controlcircuit. Since current through the rectifier SCRl must also pass throughthe safety switch heater 23, the safety switch contacts 21 and 22 willopen after approximately 8 seconds into a lock-out condition. The onlyway to start theburner again is by depressing the manual reset plunger110. An open circuit in the rectifier SCRl will render the controlcircuit inoperative since no starting current is provided in the coilRC2. The burner will thus never start. A short circuit from the gate tothe cathode of the rectifier SCRl has the same effect as an open circuitbetween the anode and cathode of SCRl. An open circuit from gate tocathode of the rectifier SCRl also has this effect.

Failure of the diode D1 in the short circuit state causes AC voltage toappear across the capacitor C2 and since AC voltage is destructive tothe capacitor C2 it will generally cause it to fail short circuited.Hence, there is no coil power to the reed switch coil RC1 and the reedswitch is incapable of holding. The burner would then becomeinoperative. If the diode D1 fails open circuited, there is likewise nopower to the coil RC1 and the burner becomes inoperative.

A short circuit failure of the diode D2 reacts the same as a gate tocathode short of the rectifier SCRl as previously described. An opencircuit failure of the diode D2 will generally be destructive to therectifier SCRl and any failure of SCRl will render the control circuitinoperative as previously described.

An open or short circuit failure of the capacitor C2 will prevent thereed switch 25 from pulling in and the burner from operating. The burnerwill also be prevented from operating if either of the coils RC1 or RC2of the reed switch 25 become open or short circuited since such failurewill prevent the reed switch from pulling in and closing the contacts 26and 28.

The resistor-R1 prevents the accidental destruction of the diode D2 by aserviceman in the field. This could happen if a serviceman accidentallyshorted one of the thermostat terminals with the proper terminal of thecadmium sulfide flame detector 32. Open circuit failure would react inthe same manner as an open circuit in the flame detector 32. Shortcircuit of either of the resistors R1 or R2 would simply eliminate theprotection measure from the equipment.

The resistor R3 is a wire wound type so that short circuit failure canbe neglected. Open circuit failure of the resistor R3 would result inelimination of thermostat bias current used for conventional thermostatpreheaters. The resistor R3 plays no other role in the circuit otherthan for this home comfort feature.

Continuing the description of the fail-safe operation of the primarycontrol 10, the resistor R4 is utilized for the purpose of calibratingthe cadmium sulfide flame detector 32. If the resistor R4 is opencircuited then SCRl never receives current from gate to cathode and willnever turn on. Since the rectifier SCRl must conduct to pull in the reedswitch through the coil RC2, the burnerwill never turn on. If the burneris in the middle of a cycle when the resistor R4 fails open, then theburner will fail to start on the next cycle. If the resistor R4 fails ina short circuit condition, then neither of the coils RC1 or RC2 will beenergized and the reed switch contacts will not close so that the burnerwillbe inoperative.

The resistor R5 protects the diode D1 from current surges to thecapacitor C2 during normal operation. If the resistor R5 were to shortcircuit then the diode D1 may fail shorted and the burner would becomepermanently inoperative in the manner previously described in connectionwith failureof the D1. If the resistor R5 fails open circuited, then nopower-will be furnished to the coil RC1 and the reed switch contactswill not close. The burner would then be inoperative.

The resistor R6 functions to limit the power to the coil RC1. Theresistor R6 iscalibrated and calibrates the coil RC1 to within aspecified drop-outrange for the reed switch. As is well known, wirewound-resistors do not fail short. If open circuit failure results, thenno power is supplied to the coil RC1 and the reed switch will not pullin. The burner will thus be inoperative if the resistor R6 fails opencircuited.

The wire wound potentiometer R7 is' used to calibrate the pull-involtage of the reed switch. This is accomplished by shunting currentaway from the reed switch coil RC2. An open circuit in the potentiometerR7 allows the reed switch to pull in at lower line voltage than theset-point voltage, as for example volts. Short circuit of thepotentiometer R7 prevents power from flowing to the reed switch coil RC2and the reed switch will not close the contacts 26 and 28. The burnerwill then be inoperative.

The heating coil of the safety switch 20 cannot fail shorted. An opencircuit failure functions in the same manner as an open circuit failureofthe anode to cathode on the rectifier SCRl previously described. Withrespect to the cadmium sulfide flame detector 32, this flame detectormaintains approximately 1,500 ohms at 1 foot candle illumination. Shortcircuit results in the failure to start the burner when the thermostatcloses. An open circuit causes the safety switch 20 to lock out.

While a preferred embodiment of the invention has been illustrated anddescribed, it will be understood that various changes and modificationsmay be made without departing from the spirit of the invention.

What is claimed is:

1. In an electrical switch for. use in a primary control system forfurnaces, the combination including a housing having an end wall, aU-shaped mounting base having a web portion and a pair of spaced flangeportions disposed in said housing, first and second bimetallic blades,one end portion of said first blade being fixed to one of said flangeportions, the other end portion of said first blade being fixed directlyto one end portion of said second blade, said first and second bladesbending in opposite directions upon an increase in temperature, stopmeans carried by the other end portion of said second blade, a pair ofresilient contact members fixed to said web portion of said base, saidcontact members being in electrical contact with each other when one ofsaid members is in engagement with said stop means, adjusting screwmeans threadably engaging said one flange portion and bearing againstsaid first bimetallic blade, said adjusting screw means being effectiveto adjust the position of said stop means carried by the other endportion of said second blade relative to said one contact member,electrical heating means encompassing said first blade and beingeffective to heat said first blade to cause said stop means to bedisengaged from said one contact member responsive to bending of saidfirst blade, the directions of bending of said first and second bladesresponsive to a change in temperature being such that an increase intemperature of said second blade will tend to counteract deflection ofsaid stop means responsive to an equivalent increase in temperature ofsaid first blade, a manually operable plunger, said plunger beingsupported for axial movement by the other of said flange portions andsaid end wall of said housing. said plunger sequentially engaging saidcontact members upon the application of manual force thereto andmaintaining separation of said members until said one contact memberengages said stop means, and resilient spring means bearing against saidother flange portion and said plunger and biasing said plunger away fromsaid one contact member and toward said end wall.

1. In an electrical switch for use in a primary control system forfurnaces, the combination including a housing having an end wall, aU-shaped mounting base having a web portion and a pair of spaced flangeportions disposed in said housing, first and second bimetallic blades,one end portion of said first blade being fixed to one of said flangeportions, the other end portion of said first blade being fixed directlyto one end portion of said second blade, said first and second bladesbending in opposite directions upon an increase in temperature, stopmeans carried by the other end portion of said second blade, a pair ofresilient contact members fixed to said web portion of said base, saidcontact members being in electrical contact with each other when one ofsaid members is in engagement with said stop means, adjusting screwmeans threadably engaging said one flange portion and bearing againstsaid first bimetallic blade, said adjusting screw means being effectiveto adjust the position of said stop means carried by the other endportion of said second blade relative to said one contact member,electrical heating means encompassing said first blade and beingeffective to heat said first blade to cause said stop means to bedisengaged from said one contact member responsive to bending of saidfirst blade, the directions of bending of said first and second bladesresponsive to a change in temperature being such that an increase intemperature of said second blade will tend to counteract deflection ofsaid stop means responsive to an equivalent increase in temperature ofsaid first blade, a manually operable plunger, said plunger beingsupported for axial movement by the other of said flange portions andsaid end wall of said housing, said plunger sequentially engaging saidcontact members upon the application of manual force thereto andmaintaining separation of said members until said one contact memberengages said stop means, and resilient spring means bearing against saidother flange portion and said plunger and biasing said plunger away fromsaid one contact member and toward said end wall.